I have no experience of the 10" speakers at all, but Celestion claim that the standard G10 is voiced like a Vintage 30, and that the G10 "greenback" is like a greenback but perhaps with more bass resposnse.
Greenbacks and V30's are both great speakers. The V30 is more suited to more modern styles, and has an upper mids peak which really cuts through very nicely with heavily distorted tones.
The greenback is another mids-heavy speaker, but is better for vintage tones of the 60's and 70's - everything up to ACDC's Back in Black. Although greenbacks can also do some good higher gain tones too.
Basically, greenbacks are more "vintagey", slightly looser in the bass. V30's are tighter (and louder), with a more modern tone, and can also handle twice as much power, so your amp wattage is a real issue here too.